This invention relates generally to the accurate measurement of angular rotation and, in particular, to the performance testing of rotating machines.
Conventional approaches to the performance testing of electric motors generally measure steady-state performance of the motor after the elimination of transient effects. To this end, motor speed data is collected as a function of time and the data is filtered so as to remove fluctuations in order thereby to derive standard motor performance characteristics as set out in the appropriate IEEE standards, for example. Such characteristics invariably relate to the torque-speed performance of the motor at no load whereby the motor is accelerated from rest under no load conditions, and the torque is derived as a function of the time derivative of the speed curve in accordance with Newton""s Second Law of Motion. Additionally, so-called xe2x80x9csignaturexe2x80x9d tests as well as load tests are performed, although in all cases the effects of fluctuations are eliminated.
Signature testing is an extension of no-load testing that utilizes faster measuring techniques and processing in order to compare a specific motor""s no-load performance with that of a pre-calibrated xe2x80x9cmasterxe2x80x9d motor which serves as a yardstick against which production line motors may be assessed. Load tests measure the motor""s performance under operating conditions whereby a specific torque is applied to the running motor under test, and the resulting speed, current and power are measured.
Typically, the motor speed is measured using a tachometer coupled to the motor""s axis. This allows acquisition of the motor speed in analog form and suffers from low resolution and severe noise contamination. For this reason, digital methods are preferred and significant effort has been expended during the last two or three decades to allow more accurate digital sampling of the rotational velocity of motor shafts and the like. Many such methods still employ what are essentially analog transducers to derive the speed signal and then digitize the speed signal using AID converters, so as to allow subsequent processing to be performed digitally.
R. Szabados et al. describe such a technique in xe2x80x9cMeasurement of the Torque-speed Characteristic of Induction Motors using an improved new digital approachxe2x80x9d appearing in Transaction on Energy Conversion, Vol. 5, No. Sep. 3, 1990. Their method uses a fast data acquisition system to sample the output of a d.c. tachometer as well as other relevant parameters such as line current and voltage. The measured data is then processed digitally to remove the noise, perform dynamic average filtering to eliminate extraneous coupling vibrations and to determine the relative torque profile from the time derivative of the speed curve using Newton""s Law. Since the removal of noise also eliminates the fluctuations, it thus emerges that the elimination of fluctuations by filtering the raw speed data is an inherent feature of the method proposed in this article.
Indeed, it is further shown that the raw speed data is contaminated and that the first task of the data processing phase involves removing the extraneous signals without distorting the speed profile. A major contribution of the above-referenced paper resides in the improved filtering algorithms which are presented.
U.S. Pat. No. 5,218,860 (Storar) assigned to Automation Technology, Inc. discloses an alternative approach wherein, rather than measure the speed using analog transducers, a digital gray scale (incremental) encoder is used. FIG. 1 shows pictorially a motor test bed 10 wherein a motor is mechanically coupled to a test system according to U.S. Pat. No. 5,218,860 via a test fixture consisting of a rotating shaft 12 supported on high-quality bearings 13. Mounted on the shaft 12 are a flywheel 14 of known inertia and a high-resolution rotary digital encoder 15. The flywheel 14 acts as an inertial load whereby torque may be determined according to the equation:                     T        =                  I          ·                                    ⅆ              v                                      ⅆ              t                                                          (        1        )            
where: T=Torque,
I=Inertia of flywheel,
v=speed, and
t=time.
As explained in U.S. Pat. No. 5,218,860, the torque-speed characteristics are sampled at regular known time intervals during the time it takes for the motor to reach full speed from standstill. The measurement time interval is fixed by a crystal oscillator and is usually 16.67 ms, corresponding to the period of one 60-Hz power line cycle. The change in speed is determined by the rotary encoder which resolves as little as 0.0072xc2x0 of angular displacement. Torque and speed are computed for each 16.67 ms period from the time power is applied until it reaches its maximum no-load speed. The inertia of the flywheel attached to the motor is so selected that the motor reaches full speed in about 4 seconds, this being the time it takes to sample some 240 torque and speed results, enough to describe the entire torque-speed curve from standstill to full speed.
The digital rotary encoder as described U.S. Pat. No. 5,218,860 offers a significant improvement over analog transducers, and allows the measurement of certain motor characteristics that were previously not easily obtained. However, the resolution of the device is still relatively poor since, in effect, a very large number of pulses are averaged during each sample time period. Specifically, it is stated in U.S. Pat. No. 5,218,860 that the incremental encoder produces 25,000 pulses during each full rotation of the motor shaft. Assuming an average motor speed of 10,000 rpm, this means that the number of pulses produced per 16.67 ms time period is nearly 70,000. The actual number of pulses is counted by a binary counter so as to provide an accurate indication of the angular speed of the motor. However, during a sampling period as large as 16.67 ms, the fluctuations are no longer measurable: thus allowing only the smoothed characteristics to be determined. Moreover, there would appear to be no particular advantage in employing a rotary encoder having such a high resolution, as well as cost, given that such a coarse sampling interval is employed. Theoretically, the resolution could be improved simply by sampling at smaller time periods. However, in practice it is difficult to achieve this accurately and inexpensively using current technology.
Moreover, the flywheel attached to the motor shaft loads the motor and, whilst this does not derogate from the static performance of the motor, it substantially eliminates the fluctuations to which the transient effects are subjected. Consequently, loading the motor as taught in U.S. Pat. No. 5,218,860 does not allow measurement of the dynamic performance of the motor.
The present inventor has found that the dynamic performance of the motor provides invaluable information about the motor, such that without a knowledge of the dynamic performance of the motor it is impossible to derive fundamental behavior of the motor. However, for the reasons set out above, the dynamic performance data is unattainable during a sample period as large as 16 ms, since during this time period most of the fluctuations on the transient part of the curve are lost. Even regardless of the actual magnitude of the sampling time period, and bearing in mind that some improvement can clearly be achieved by reducing the sampling time period conducive with prevailing technology and price constraints, any improvement is limited in scope. This follows from the fact that counting pulses during a fixed time period, however small, can never allow optimum results to be achieved. Thus, even if the sampling time period could be reduced indefinitely (which, of course, it cannot) it can never be reduced to less than the period of a single pulse since, in such case, no data would be obtained during the sample period. On the other hand, whilst increasing the sampling time period ensures that sample data will be obtained, it does so at the cost of producing multiple data per sample. This means that the resolution thereby obtained is inevitably less than the theoretical maximum.
Furthermore, in order to obtain a sufficient number of sample points using the approach disclosed by U.S. Pat. No. 5,218,860, it is necessary to ensure that the time required for the motor to attain full speed is extended to several seconds. This is achieved by means of the flywheel having sufficient inertia to delay the steady state response of the motor. It would obviously be preferable to allow the relevant speed characteristics to be derived in less time without, of course, compromising on the number of sample points and, at the same time, providing details of the fluctuations on the transient performance which disappear when the motor is loaded.
U.S. Pat. No. 4,535.288 to Joseph L. Vitulli, Jr, discloses a method for determining the rotational speed of a moving shaft in a spatially limited environment, wherein the time between a sequentially successive pair of encoder (transducer) pulses is used to determine the speed. Updated rotational speed is computed from a flier pair of sequentially successive pulses, which are non-sequential to the earlier pulses. The rotary encoder described by Vitulli can be likened to a toothed wheel having sixty equally spaced teeth, each of which gives rise to an output signal having a first voltage level when rotating past a pickup. When the space between adjacent teeth passes the pickup, an output signal having a second voltage level is produced Typically, the first and second voltage levels translate to digital signals having logic HIGH and Low levels, respectively, such that a pulse train is produced. Thus, assuming that there are provided sixty equally spaced teeth which give the same angular for the high and low levels, the angle of rotation corresponding to each logic HIGH level is 2xcfx80/120 radians. By measuring the time duration of each logic HIGH level, the angular speed can thereby be calculated.
In practice, however, even the best rotary encoders have a duty cycle accuracy of only xc2x110%, which means that whilst the distance between the Start of adjacent teeth (corresponding to the period of the pulse train) is constant, the width of each tooth is subject to an accuracy of xc2x110%. Since angular speed is calculated on the basis of the measured time for each tooth passing the pickup, it is clear that his is dependent on the actual width of each tooth and is thus subject to a maximum error of 20%.
JP 59 160766 (Fanuc) discloses a speed detecting device of a servo-motor is determined using a rotary encoder in a manner similar to that described in U.S. Pat. No 4,535,288 and thus subject to he same problems of inaccuracies owing to duty cycle errors. The is moreover no suggestion to test the marine when unloaded.
GB 2 127 549 discloses a test bed for supporting a motor during measurement of the motor""s torque. The system disclosed thereby appears to be very similar to U.S. Pat. No. 5,218,860 discussed at length above and is subject to the same deficiencies. Specifically, it is to be noted that the motor whose steady-state and transient torque are measured by GB 2 127 549 is loaded in order to reduce the acceleration of the motor (loading the motor causes all the dynamic phnomenon spoken at the present application to disappear). It is clear from the description in GB 2 127 549 on page 1 lines 48 to 53 that such loading is necessary in order to record the transient torque-speed characteristics of the motor starting from zero speed to full speed. It will be apparent that reducing the acceleration of the motor as suggested by GB 2 127 549 militates against the derivation of the motor""s transient characteristics whose correct determination is an essential feature of the present invention. U.S. Pat. No. 4,169,371 (Witschi et al.) discloses a method and apparatus for measuring the torque and/or power of a drive system in dynamic operation based on the time differentiation of the speed of the drive to determine acceleration. The system is loaded and it is therefore apparent that the dynamic characteristics whose determination is the principal objective of the present invention are lost.
U.S. Pat. No. 5,631,411 (Harms et al.) discloses an He monitoring apparatus that calculates the speed of a motor. It is clear from FIG. 1 that an inertial load (i.e. a flywheel) is conned to the motor and therefore here, too, the dynamic characteristics whose determination is the principal objective of the present invention are lost.
EP 457 086 discloses an apparatus for the contactless measurement of the local dragged-in torque in a worm machine. At least two position sensors or proximity switches are arranged in the worm casing. During the rotation of the worm shank, the sensors scan the worm shank surface and, on the basis of detected characteristics, generate measurement pulses which, together with a speed signal, are suppliable to an electronic analysis circuit, which calculates the local dragged-in torque in a segment of the worm shank, within the product space. The apparatus operates in conjunction with a device for measuring the integral torque being arranged between the worm shack drive and the product space of the worm machine. There is no suggestion here either to measure torque of an unloaded machine.
U.S. Pat. No. 5,390,545 (Doan) discloses an apparatus for measuring torsional vibrations of rotating machinery wherein a wheel having a plurality of spaced apart teeth is connected to the rotating machinery. A sensor detects the speed of wheel rotation and responsively produces a speed signal that has a frequency proportional to the rotational wheel speed. A timing deice receives the speed signal, determines the period of the most recent pulse of the speed signal, and responsively produces an instantaneous period signal that has a value representative of the determined period.
U.S. Pat. No. 4,992,730 (Hagiya) discloses a method of computing the rotating speed of a rotating body by setting speed computation reference time periods with respect to a pulse train signal obtained from the output of a rotating speed sensor; measuring time length from the last pulse edge in the previous speed computation reference time period to the last pulse edge in the current speed computation reference time period; and computing the rotating speed of the rotating body on the basis of the result of the time length measurement.
It is therefore an object of the invention to provide a method and system for measuring rotational speed in which the drawbacks associated with hitherto-proposed are substantially improved or eliminated.
Such an objective is realized in accordance with a broad aspect of the invention by providing a method for measuring an angular rotation of a rotating shaft, the method characterized by the steps of:
(a) attaching to the shaft a digital rotary encoder which successively generates opposing binary logic states such that any pair of sequential logic states corresponds to a known angular rotation of the shaft, rotating the shaft,
(c) separately measuring a respective time period associated with each successive logic state generated by the digital rotary encoder, and
(d) surnming said respective time periods associated with each successive logic state so as to derive an accumulated elapsed time interval of successive pairs of logic states generated by the digital rotary encoder thereby allowing derivation of the angular rotation or a function thereof of the shaft.
The invention thus allows an improved approach to testing motor or engine speed according to the time that elapses during a known angular rotation of the shaft. According to such an approach, the elapsed time is measured for the logical states to change from LOW to HIGH and back to LOW or vice versa. Although the tie interval during which the logic state remains either LOW or HIGH is subject to duty cycle error, the combined time interval for sequential logic states is an accurate reflection of a known angular rotation. Thus measuring the accumulated elapsed time interval between successive pairs of logic states avoids duty cycle errors affecting the speed results whilst reflecting any change in speed on the fly. Consider, for example, a rotary encoder that produces 60 pulses per revolution. In the case of U.S. Pat. No. 4,535,288, the shaft revolutions per minute (rpm) can be determined in a one second degree interval, and in the case of a very high quality encoder (with duty cycle error in the order of xc2x110%) will give rise to a measured speed inaccuracy of xc2x110%.
The method according to the invention finds particular application for testing an electric motor or a component thereof the method characterized by the steps of:
(a) attaching an unloaded shaft of the electric motor to a digital rotary encoder which generals opposing binary logic states such that any pair of sequential logic states corresponds to a known angular rotation of the rotating electric motor,
(b) measuring an accumulated elapsed time period of successive pairs of logic states generated by the digital rotary encoder during rotation of the electric motor so as to allow derivation of a dynamic speed-time characteristic of the rotating electric motor or a function thereof, and
(c) using the dynamic speed-time characteristic of the unloaded rotating electric motor to derive static torque speed or dynamic torque speed or oscillating torque during steady state or speed and torque specturn during steady state of the unloaded rotating electric motor. Preferably, there is further included the step of:
(d) calculating a torque of the rotating machine by reference to a predetermined moment of inertia of a rotor hereof and the measured speed characteristic of the rowing machine.
The invention also contemplates an apparatus for deter dynamic and static speed-time, torque-time and speed-torque characteristics of a rotating machine or of a component thereof. By using a pre-calibrated rotor, tests may be performed on identical machines using different stators so as to provide relative performance data (both static and dynamic) of the different stators. Likewise, using a pre-calibrated stator, tests may be performed on identical machines using different rotors so as to provide relative performance data (both static and dynamic) of the different rotors.
It will thus be understood that the method and apparatus according to the invention allow dynamic and static performance data to be derived without requiring the connection of an external inertial load to the machine""s axis. This allows the machine to reach steady state (i.e. non-transient) operation more quickly and allows calibration of the machine to be effected more quickly. This is of particular importance when small machines are mass-produced and must be tested on the production line. Moreover, it allows the measurement of fluctuations, which have hitherto eluded measurement.
It is admittedly suggested in U.S. Pat. No. 5,218,860 [col. 1, line 29] that for larger motors, the mass of the armature may be sufficiently large to provide a proper inertial load. That is to say U.S. Pat. No. 5,218,860 also allows for the external inertial load to be dispensed with, albeit for large motors only. However, this may be only be done because large motors, being inherently inertial, in any case take a relatively long time to reach steady state speed, thus allowing sufficient sample points to be obtained. This is not the case for small low-inertia motors where the external inertial load is mandatory in U.S. Pat. No. 5,218,860 in order deliberately to slow down the time to reach steady state speed and thus allow sufficient sample points to be obtained. It is thus clear that U.S. Pat. No. 5,218,860 does not allow extrapolation to the present invention which allows for the flywheel to be dispensed with even for small motors, since a major object of the invention is to reduce, rather than increase, the time to reach steady state speed.
The invention also allows measurement of oscillating torque and speed during steady state condition so as to derive speed-time and torque-time characteristics in both the time and frequency domains. In such case, a flywheel may be used to slow down the time for the machine to reach steady state, thereby producing steady state oscillating torque and speed phenomena during the acceleration. This allows faults with the machine to be highlighted that would not otherwise be apparent.
The invention also permits greater flexibility of testing the rotating machine. A user can control the sampling time and the time from which the sampling begins. The user can likewise control x-axis (time and frequency) and y-axis (torque and speed) thus allowing the device to be used as a rotating machine analyzer.